Hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets that are widely used in the fields of automobiles, transport aircrafts and the like, are required to have, in addition to high strength, also excellent formability in terms of bendability and hole expandability (stretch flangeability), and to exhibit moreover excellent delayed fracture resistance. Such steel sheets are further required to have excellent impact absorption properties.
Adding a substantial amount of strengthening elements such as Si or Mn into steel is effective in order to secure high strength and formability. However, Si and Mn are readily oxidizable elements, and hot-dip galvanizing wettability is significantly impaired by an oxide film, having Si oxides, Mn oxides and complex oxides of Si and Mn, that is formed on the surface. This poor wettability gives rise to problems such as bare spots and the like.
Various technologies have therefore been proposed with a view to enhancing the formability and so forth of plated steel sheets that contain large amounts of Si and/or Mn.
For instance, Patent Literature 1 discloses a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or higher and being excellent in bendability and corrosion resistance in worked portions. In further detail, Patent Literature 1 discloses the feature of significantly speeding up the growth of a decarburized layer with respect to the growth of an internal oxide layer that is formed from the interface of the steel sheet and a galvanized layer towards the steel sheet, so as to enable suppression of bending cracks and damage to a galvanized coating that are caused by the internal oxide layer. Further, Patent Literature 1 discloses a surface-near structure in which the thickness of the internal oxide layer in a ferrite region formed by decarburization is controlled so as to be thin.
Further, Patent Literature 2 discloses a hot-dip galvanized steel sheet excellent in fatigue durability, resistance to hydrogen embrittlement (synonymous with delayed fracture resistance) and bendability, the steel sheet having a tensile strength of 770 MPa or higher. In further detail, a steel sheet portion in Patent Literature 2 is configured to have a soft layer directly in contact with the interface with a galvanized layer, and a soft layer in which ferrite is set to be the structure of highest area ratio. Further, Patent Literature 2 discloses a hot-dip galvanized steel sheet in which a thickness D of the soft layer, and a depth d, from a galvanized layer/base iron interface, of an oxide that contains one or more from among Si and Mn and that is present in a surface layer of the steel sheet, satisfy d/4≤D≤2d.